Heat the margarine in a large soup pot. Add the onion and celery and sauté over moderate heat until the onion is golden. Add the potatoes, cauliflower, and enough water or stock to barely cover. Stir in the curry and mustard. Bring to a boil, then cover and simmer until all the vegetables are tender, about 20 to 25 minutes. Remove from the heat.

With a slotted spoon, transfer half of the solid ingredients to the container of a food processor. Process until smoothly puréed. Stir back into the soup pot with the remaining soup. Add just enough milk to achieve a slightly thick consistency. Return to low heat and bring to a gentle simmer. Sprinkle the cheese in a bit at a time, stirring in each batch until thoroughly melted. Stir in the peas and dill and simmer over low heat for 10 minutes, stirring occasionally.

Adjust the consistency with more milk if necessary and season to taste with salt and pepper. Let stand several hours, then heat through before serving.